Protective circuit



May 11, 1954 L.' s. LAPIN 2,678,391

PROTECTIVE CIRCUIT Filed July 2, 1952 IN VEN TOR.

fier .flap/vw MH M ,frrbRNEY Patented May 11, .1954

PROTECTIVE CIRCUIT Lester S. Lappin, Merchantville, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application July 2, 1952, Serial N0. 296,801

16 Claims. l

This invention relates to protective circuits, and more particularly to a combined screen grid power supply and protective circuit for screengrid-type tubes. This invention has particular application in transmitters, wherein the screengrid-type tubes used (for example, tetrodes) are rather large and have high power-handling capabilities. v

Certain advantages are obtained through the use of electron tubes containing screen grids. However, these tubes are easily damaged by the application of screen grid voltage thereto in the absence of anode voltage, since in this case the screen grid current usually becomes excessive due to the fact that the electron ow is entirely to the screen grid, rather than only partly to such grid. When the screen grid current becomes excessive, damage to the tube will result. If the cost of the tube is high, it is quite desirable to provide some means for protecting the tube against such damage.

An object of the present invention is to devise a' novel screen grid power supply circuit for screen grid vacuum tubes. Y

Another object is to devise a screen grid power supply circuit which operates to protect screen grid tubes against damage which occurs when voltage is applied to the screen grid of such tubes in the absence of anode voltage.

A further object is to provide a screen grid power supply circuit which prevents application of screen grid voltage to a screen grid tube in the absence of anode voltage on such tube.

Yet another object is to devise a screen grid power supply circuit which operates to delay the application of screen voltage to a tube for any predetermined time after the application of anode voltage to such tube.

The foregoing and other objects of the invention are accomplished, briefly, in the following manner: The screen grid tube being protected is supplied with anode potential from an anode power supply and is supplied with screen grid potential through a series regulator tube from a separate screen grid power supply. The regulator tube is ofthe screen grid type, the screen grid of this tube being supplied with potential from the main anode power supply through a voltage divider network provided with a time delay feature. Regulation of the screen grid potential supplied to the protected tube is elected by means of a direct current amplier suitably coupled to the series regulator tube.

The foregoing and other objects of the invention will be best understood from the following description of an exemplication thereof, reference being had to the accompanying drawing, wherein: y

Fig. 1 is a simplified schematic of a circuit according to this invention; and

Yco

Fig. 2 is a detailed schematic of a practical embodiment of this invention, such as might be used in a television transmitter.

Now referring to Fig. l, the evacuated electron discharge device I, which is a screen grid tube of the tetrode or pentode type, is the tube being protected by the circuit of this invention. Tube I has at least four electrodes including an anode 9, a screen grid IQ, a control grid II, and a cathode I2. Tube I is illustrated as a tetrode, but it may be a pentode or other type tube having four or more electrodes, including a screen grid. Tube I may be, for example, the final or output tube of a radio frequency power amplier in a transmitter, and may have high power-handling capabilities. Such a tube is likely to be rather costly and it is therefore desirable to protect such tube against appreciable damage to its screen grid arising from excessive screen current, when the anode potential on this tube goes off for any reason. Y

Anode potential is applied to tube I from an anode power supply 2 by connecting the anode 9 to the positive anode power supply lead and the cathode I2 to ground, the negative power supply lead being grounded as shown.

' Screen grid potential for tube I is obtained from a separate screen grid power supply 3 through the anode-cathode path of a series regulator vacuum tube 4. The positive screen power supply lead is .connected to the anode I3 of tube 4, while screen grid I0 of tube I is connected to the cathode I4 of tube 4. The negative screen power supply lead is grounded. Thus, the screen current of tube I flows through tube 4 and the potential on screen I is the same as that of cathode I4 of tube 4.

A direct current amplifier 5, having a voltage gainon the order of 5,000 times, has its input connected to cathode I4. of the regulator tube and its output connected to the control grid I5 of this tube. This amplifier operates in a man ner to be further detailed hereinafter, in such a way that when the potential of cathode I4 (and of screen grid IB) tends to fall, the voltage applied to regulator control grid I5 changes in such a direction as to cause the potential of cathode I4 to rise. The converse action occurs when the potential of cathode I4 tends to rise. In other words, the D. C. amplifier 5 in conjunction with the regulator tube 4 causes the potential on screen grid ID of tube I to be constant, within the range of regulation of tube 4. This of course assumes that tube 4 is conducting and that the circuit is otherwise in normal operating condition.

Tube 4 is a screen grid tube the screen potential for which is obtained through a voltage divider from the anode power supply 2. Thus, a pair of resistors 6 and I6 are connected in -divider 6, I6.

lI'I with respect to cathode I4 is zero, dueto-the Vnon-conductive. `deterrninedbynthe time constant of :thezvoltage series between the positive anode power supply lead and ground, and the screen grid Il of tube 4 is connected tothe junction `of these twore sistors. The voltage divider TIi, I reduces the potential of power` supply 2 to a suitable value for application to screen grid Il. A capacitor 8, which is connected from the junction of resistors and IE5 to ground, serves to "delay the build-up of screen potential .fonscreenfll A gaseous voltage regulatortube 'I is :connected from screen grid I'I to cathode I4 of regulator tube 4 and serves to maintain the ,screen potential of tube 11 constant after Ythe'voltage across tube 'I has increased suiciently to cause-breakdown of this gaseous discharge tube. Ater tube 'I `breaks down.. its anode-to-cathode potential is constant. `In this way, variations inthe voltage of anode power suppl-y 2 are prevented from causing any variations in the output voltage (screen supplyfor tube I) of tube 4. Also, the screen potential of tube 4 is maintained at the correct value for` proper operationk of this tube.

To describe the operation of the-Fig. l circuit, let us assume that the screen power supply 3 is energized before theanode powersupply l2. It should be apparent that, if the-opposite is the case,no screen voltage canvpossibly be supplied to screengrid I until powersupply 3 is energized, since until this occurs there cannot be any appreciable potential at cathode I4 ofregulator tube 1I (which tube is in series between power supply 3 and screen I0). If screen supply 3-is energized before anode supply 2, voltage is applied to theanode I3 of tube 4. However, the screen grid I'I is atzero1potential, so no current can ow through tube 4 at this time. Now, if anodepowerv supply 2 is energized, a positivefpotential is applied to anode v9 Vand to the voltage Initially, the -potential of .screen presence of capacitor 8. Therefore, vtube `II is 'As capacitor I3l chargesiat a. rate div-ider, I6 land capacitor 8, .the potential on screen grid ITI gradually increases andthe-anode vcurrentof tube 4 and screencurrent'of'tube /I gradually increase (the screencurrent-'of tube V'I tion prevents any further increase in vvoltage,

vdue vto theiaction of amplifier in theregulator circuitpreviously described. After the discharge in tube 'I is initiated, itsanode-to-cathode po 'tential is constant, so that the potential on screen grid I I is then constant. Normal operating conditions have nowbeen reached. Thus, the ap- `plicationof screen voltage totube I via lead |29 .is delayed by any predetermined time .after t'he application of anode'voltage thereto.

If the anode supply voltage goes Vorf lfor some reason after operating conditions have been reached, the potential on screen grid. I1 of series tube 4 falls to zeroalmost instantly, sincefsuch screen lgrid potential is obtained from anode power supply 2. This cuts 01T tube '4, so'thatthe potential onscreen grid I0 'of tube I -drops immediately. Thus, theapplication offscreen voltagetotube I is preventecbthe absence `of Aanode supply voltage keeping tube 4 from'conducting. Since the screen `potential of tube VI is '-.thus dropped, .the screen current of this'tube is'greatly reduced, to a value'much less thanthe normal operatingscreen current. -Thezscreen current of l5 used in a television transmitter.

`"ducedtwhen-theanode voltage goes off, damage to tube Iwl'iich might result if this were not done, is .effectively prevented.

Fig. 2 is a schematic diagram of a practical .embodiment of this invention, such as might be In Fig. 2, parts similar tothose of Fig. 1 are denoted by the rsa-rnereference numerals.

-As in Fig. 1, the evacuated `screen grid tube I (shown asa tetrode) lis the tube tov be protected,

.and the anode 9 -andcathode I2 'of thistube are I0 of tube I.

fconnected across theanode power supply 2. In fFig. .2, four'series regulator vacuum tubes 4 in electrically parallel relation are used between the fscreen -grid power'supply 3 and the screenfgrid The anode I3 :of-each tube 13 is '--connected directly to the positive output lead of screen power supply 3,1whilethe cathode 'I-of each tube `II is vconnected through a separate l.balancing resistor numbered I8 to thehcommon -screen grid potential routput lead I9 which is connected to screen grid I0 of tube I.

In orderto-'supply screen gridfpotential to the regulatortubes 4, a supply-lead 2is connected to the junction of resistors 6 and I6 l(the voltage 135 divider -which is V`connected across VA'anode power supply 2) and a separateparasiticsuppressing :resistor 2| 'is 'connected from this leadto'each respective regulatortube screen grid I 1. The screen potential of tubes 4i is maintained constant 10 1by `a gaseous voltage regulator tube 'I 'connected -between leads 20 and I9.

The D.` C'. amplifier `5 consistsv of'a twin triode yacuumtube `22fand its circuit connections, now to be described. The input to this "amplifier is ztaken-from the regulator tube cathodelead I9 70 22 `and alead l33.

`through. a voltage divider comprising 'resistors 23 fandf25. Cathode`24 is connected to the junction of resistorsk23'and 425^atwhich"point the voltage ris proportional `to the potential 'on vthe screen =501grid I 0. Any variation in fscreen grid potential thusappears atthe inputv of the D. C.'amplier I(cathode 24). Asubstantially constant bias is provided on the right-hand control grid 27 of tube22 "b5/,means of'a potentiometric tap con- M 4rnected tofsaid grid and movable on the central 'one "3U-of three series-connected"resistors '29, "39, -and 3| -arranged betweenthe left-hand lcathode .32 .oftube-ZZ and theupper, ungrounde'd end of .resistor v'26. The voltageVv across the -seriesfcom :50 bination 29-3I is maintained constant by `means .of fafgaseousvoltage regulator tube`33.

The'tube .122 is arranged as a two-stage D. C. amplifier, withits input at the cathode 24. An `anodetloadresistor 3d.' is connected between Ythe :right-hand anode `35 of tube 22 and lead I9.

/Anode A35 is directly coupled to the left-hand control grid 3'6 of -tube 22, while the output of D.1C..amp1ifier5 is-'taken off by means of aconnection between the'left-"hand anode '3T oftube VThe control 'grid I5 or' Veach regulator tube'd isl connected through a'respective parasitic` suppressing resistor 39 to'lead i38.

The D. C. amplifier 5 operates, in 'conjunction with fregulatorftubes 4, to maintain the screen potential on screen grid LIII of tube 'I Ysubstan' tially constant, when the circuit is in normal operating condition. Any incipient change of voltage on screen grid potential output lead I9 is applied to the cathode 24 input of D. C. amplifier 5. The bias voltage on control grid 2l is held substantially fixed by means of voltage regulator tube 33 connected across resistor network 29-3I, the lower electrode of tube 33 being connected to the ungrounded end of resistor 2'6 and the upper electrode of this tube being connected through a resistor 40 to lead I9. The voltage change across the first stage anode load resistor 34 is 'applied to the grid 36 of the second stage amplier, and the voltage change across the second stage anode load resistor 4I (connected between anode 31 and the positive lead of power supply 3) is applied to the control grids I5 of the respective regulator tubes 4. The connections are such that if the potential of lead I9 tends to increase, the voltage applied'to grids I5 by way of amplifier 5 will be of such a polarity as to decrease this lead potential, while if the potential of lead I 9 tends to decrease, the voltage applied to grids i5 by way of amplifier 5 will be of such a polarity as to increase this lead potential. Thus, the series regulator tubes 4 operate in conjunction with the D. C. amplifier 5 to maintain the screen potential of tube I quite constant.

The operation of the Fig. 2 circuit, to protect tube I from the possibility of damage due to excessive screen currents when the anode voltage goes oi this tube, is exactly like that of Fig. 1, previously explained, so the description will not be repeated at this point. The Fig. 2 circuit is in all essential details exactly the same as that of Fig. 1. Suffice it to say at this point that when the anode voltage goes oi tube I for any reason whatever, the screen voltage is also removed from tube I. Also, the application of screen voltage to f tube I is delayed for any predetermined time after the application of anode voltage to this same tube.

The following values for certain of the components are given by way of example only, for a circuit according to Fig. 2 which was actually built and successfully tested. The circuit of Fig. r2 was actually embodied in a television transmitter designed to have a power output of 10 kilowatts and operating in the VHF television What is claimed is:

1. A power supply circuit for a tube of the screen grid type comprising an anode power supply, a separate screen grid power supply, means coupling the anode of said tube to the positive terminal of said anode power supply, an electron discharge device of the screen grid type havingA said cathode electrode to the screen grid of saidr tube, and means for applying a positive bias potential derived from said anode power supply to said screen grid electrode.

2. A circuit in accordance with claim 1, wherein the positive bias potential applied to the device screen grid is derived from a voltage divider v connected across the anode power supply.

3. A circuit in accordance with claim 1, wherein voltage regulating means is connected between the device screen grid and the said cathode, to

maintain substantially constant the bias 'poten-l tial applied to such screen grid.

4. A power supply circuit for a tube of th i screen grid type comprising an anode power supply, a separate screen grid power supply, means coupling the anode of said tube to said anode power supply, an electron discharge device hav, ing an anode electrode, a cathode electrode and two control electrodes, means connecting the anode-cathode path of said device in series between said screen grid power supply and the screen grid of said tube, connections for applying bias potential to a iirst one of said control electrodes from said anode power supply, and a voltage amplifier having its input coupled to said cathode electrode and its output coupled to the second one of said control electrodes to maintain the voltage applied to the screen grid of said tube substantially constant.

5. A circuit in accordance with claim 4, wherein the electron discharge device is a device of the screen grid type and wherein the rst one of said control electrodes is the screen grid thereof.

6. A circuit in accordance with claim 4, wherein the electron discharge device is a device of the screen grid type, wherein the first one of said i control electrodes is the screen grid thereof, and wherein voltage regulating means is connected between the device screen grid and the said cathode, to maintain substantially constant the bias potential applied to such screen grid.

7. A power supply circuit for a tube of the screen grid type comprising an anode power supply, a separate screen grid power supply, means coupling the anode of said tube to said anode power supply, an electron discharge device having an anode electrode, a cathode electrode and two control electrodes, means connecting the anode-cathode path of said device in series between said screen grid power supply and the screen grid of said tube, connections for applying bias potential to a first one of said control electrodes from said anode power supply, meansl in said connections for preventing the application of bias potential to said first control electrode lfor a predetermined interval following energization of said anode power supply, and a voltage ampliiier having its input coupled to said cathode electrode and its output coupled to the second one of said control electrodes to maintain the voltage applied to the screen grid oi' said tube substantially constant.

8. A circuit in accordance with claim 7, wherein the electron discharge device is a device of the screen grid type and wherein the first one of said control electrodes is the screen grid thereof.

9. A circuit in accordance with claim 7, wherein the electron discharge device is a device of the screen grid type, wherein the first onefofzz.,'saidl control; electrodes is the: screen grid-L theremi;` andfwhereine voltage. regulating means. 1sconnectedeibetween' the devicel screen grid 'and the said:l cathode,` to. maintain substantially constantthe biaspotentia-l applied to such screen grida 10; Aicircuit inaccordancewith claim 7', whereininthermeans for `preventing the'application of bias potential comprises av capacitorarranged to beizcharged'from ithef'anodez power ,supplyv and to whiclrtthenrstone of saidicontrol` electrodes is coupled;

11. Aecircuitiirr'accordancewith claim 7, whereinntheeelectron"discharge, device is a device of thexsoreenigrid.type, wherein the rst one of said controlitelectrodes is-th'ei'screen grid thereof, and wherein the means for preventing the application of bias potential comprises a. capacitor arrangedito:becliargedrom the anode'power supis:.zcoupled:`

v 1'2..- A circuit inaccordanceiwith' claim 7, wherein'.thefelectron:dischargeldevice. is a` device ofY thescreen grid type-wherein the first oneof said controla electrodes is.- the screen grid thereof, wherein' .the means for preventing the. applicationA offbiasspotential comprises a capacitor arrangedrtobecharged froml the anode power supplyiand towhiclrthescreen grid of said device is-vcoupledyand wherein voltage regulating means is lconnect'edibetween the "device screen grid` and thesaid cathode, to maintain Asubstantially constantxthefbiasupotential..applied to such screenl grid.l

13. A power supply circuit for aA tube of the screen 'gridtypecomprising: an anode power supply, afseparatescreen' grid kpower supply, means coupling theanode of said tube to saidanode power.'y supply, an electron discllargel device of the'fscreengrid type having at least an anode electrode, a cathode electrode and' a screenA grid' electrode, means connecting the anode-cathode path of. saiddevicein series Ibetween said screen grid power supply andthescreen grid of said tube,l connections forapplying Vbias vpotential to said'l screen grid eiectrodefrom said anode power' supply, and meansf-insai'd connections for preventing` the'. application'of bias potentialltosaid screen lgrid ielectrode Lfor a predetermined 'inter v valifollowing energization ciV said anode power supply.

l'lrApower.supplycircuit for a-tubeof the screenf gridvtype comprising an` anodeVY power supply,4 afseparate screen grid power supply, means couplingf'they anode of said' tube lto said vanode 8 i ventthe' applicationzof bias potential ;to;:said con.- trol electrode f for a; predetermined interval following e'energization;ofsaid". anodepower supply;

15: Av power supply circuit-for a'Y tubexof the screen'grid. type comprising .an anode power fsup ply; a :separate-:screen gridjpowen supply, means coupling: the: anoder: of said-:tubefto:V saidr anode ply, and'lafcapacitorin said connections.:arranged` to be chargedffrom ,the anode power supply,xsaid capacitor being'- coupled totsaid controly electrode and serving to'prevent"thefapplication ofpbiasspotential .to Ysaid .'control" ielectrod'ev Ifiori va4 predeter.J minedainterva'l; following energizationv of .saidz anode. power supply:

16;` A power.: supply ficircuitzl for a'e tube, ofi theY screen grid'typefcornprising an anode-power supe plyyaxseparate screenfigrid power', supply, means-4 coupling lthefanodei of lsaid'ftubesto said anode` powerrsupply, aaplurality of electron: discharge devices ofv the vscreenxgrid: typef each: l."1'aving'an,r

anode electrode, a cathode electrode, a control electrodeand a screen 'gridelectrode means .connecting'thezanodes ofall offfsaid'l devices to,- gether, means connecting theacathodesfof all Aof saidid'evicesi together, means connecting the :parialleled anode-cathode-'paths ofall of said devices, series.: betweenfsaid: sorteer-1i` gridt-.power-v supply4 yand Vthe A'screen-:grid of: "saidgftube,rv means connecting thesoreenr grids ,ofrtalljofzsaid devicestogether, connectionsffor applying: bias potential i' to" all' of said'` idevicei. .screen`- grids from said: anode power-supply; a capacitorin.r said 4connections:

arranged to Aloefchargedfrom the anode power supply; said Vcapacitorbeing coupledtofallof said device screen grids and serving to prevent the;ap' plication of biaspotential to saidudevice screen grids '-or a:predetermined'intervalk following,r energization of'` said 'anode power. supply,means.

connecting the f control; Aelectrodes-of v all, ,of said.

devices-together, a voltage amplifier having its" input-coupled to all of *said cathode :electrodes andy itsroutput 'coupled toxall otsaid:` control; electrodes to maintain the voltage applied totheA screen grid. of said tube substantially constant, and voltage regulatingmean's' connected between. the devicescreen'grids' andthe device cathodes, to maintain .substantially constant Vthe bias potential applied to said device screen grids.

References Cited-inthe -le 'of this patent UNITED STATES PATENTS.`V

Number Name Datei 2,355,191 Vance Aug. 8,1944 2,377,500 Johnson June v5, 1945 2,443,534:4 Eglin. June 15, 1948' 2,572,832' Bernard. Oct. 30; .1951' 

